The seven transmembrane receptors (also known as heptahelical, serpentine, or G protein-coupled receptors) comprise a superfamily of structurally related molecules. Possible relationships among seven transmembrane receptors (7TM receptors) for which amino acid sequence had previously been reported are reviewed in Probst et al., DNA and Cell Biology, 11(1): 1-20 (1992). Briefly, the 7TM receptors exhibit detectable amino acid sequence similarity and all appear to share a number of structural characteristics including: an extracellular amino terminus; seven predominantly hydrophobic a-helical domains (of about 20-30 amino acids) which are believed to span the cell membrane and are referred to as transmembrane domains 1-7; approximately twenty well-conserved amino acids; and a cytoplasmic carboxy terminus. The amino acid similarity among different 7TM receptors ranges from about 10% to more than 80% and receptors which recognize similar or identical ligands generally exhibit high levels of homology. The 7TM receptors can be grouped based on their homology levels and/or the ligands they recognize. For example, the interleukin-8 receptor, the angiotensin II receptor, the thrombin receptor, the endothelin receptors, the N-formyl peptide receptor and the C5a receptor all bind peptide ligands and share 20-40% amino acid similarity.
7TM receptors recognize a great diversity of ligands (for example, light, odorants, neurotransmitters, peptide hormones and small molecules) and transduce their signals via heterotrimeric guanine nucleotide-binding proteins (G-proteins) effecting a broad array of biological activities (including visual excitation, olfactory reception, and neurotransmission) through various intracellular enzymes, ion channels and transporters. Signal transduction pathways have been elucidated for rhodopsin [Khorana, J. Biol. Chem., 267: 1-4 (1992) and Stryer, J. Biol. Chem., 266: 10711-10714 (1991)] and the beta-adrenergic receptors [Dohlman et al., Ann.-Rev. Biochem., 60. 653-688 (1991)] and are thought to illustrate the pathways utilized by other 7TM receptors. Each 7TM receptor is predicted to associate with a particular G protein at the intracellular surface of the plasma membrane. The binding of the receptor to its ligand is thought to result in activation (i.e., the exchange of GTP for GDP on the .alpha.-subunit) of the G protein which in turn stimulates specific intracellular signal-transducing enzymes and channels. Thus, the function of each 7TM receptor is to discriminate its specific ligand from the complex extracellular milieu and then to activate G proteins to produce a specific intracellular signal. Cotecchia et al., Proc. Natl. Acad. Sci. USA, 87: 2896-2900 (1990) reports that the intracellular loop of the third transmembrane domain of the 7TM receptors comprises important determinants for receptor coupling to specific G proteins, however, Lefkowitz, Nature, 265: 603-604 (1993) summarizes reports that other regions of 7TM receptors may also be essential in maintaining 7TM receptors in a constrained, inactive conformation until ligand binding occurs.
Recently, several 7TM receptors have been identified which recognize ligands important for immunological and hemostatic activities. Holmes et al., Science, 253: 1278-1280 (1991) describes the interleukin 8 receptor (IL8R1) as involved in neutrophil chemotaxis and Sasaki et al., Nature, 351: 230-233 (1991) reports the angiotensin II receptor (AT2R) is involved in vascular hemostasis. Similarly, the endothelin receptors [Arai et al., Nature, 348: 730-732 (1990)] regulate vasoconstriction and smooth muscle tone. The C5a receptor mediates chemotaxis, granule enzyme release and superoxide generation in vitro and appears to be involved in anaphylaxis and septic shock in vivo [Gerard and Gerard, Nature, 349: 614-617 (1991)]. Thrombin is also recognized by a 7TM receptor and is a potent activator of platelet aggregation, monocyte chemotaxis, lymphocyte mitogenesis and mediates inflammatory responses to vascular injury. The N-formyl peptide (f-met-leu-phe) receptor is responsible for neutrophil chemotaxis and activation [Thomas et al., J. Biol. Chem., 265: 20061 (1990)]. While these 7TM receptors all have peptide ligands, other 7TM receptors that recognize small organic compounds also mediate proinflammatory activities. For example, the Platelet Activating Factor receptor recognizes a bioactive phospholipid [Honda et al., Nature, 349: 342-346 (1991)] which causes platelet aggregation and endotoxic shock. The thromboxane A.sub.2 receptor recognizes an arachidonate metabolite which also stimulates vasoconstriction and platelet aggregation and is implicated in stroke and bronchial asthma [Hirata et al., Nature, 349: 617-620 (1991)].
Mutations in the third intracellular loop of one 7TM receptor (the thyrotropin receptor) and in the adjacent sixth transmembrane domain of another 7TM receptor (the luteinizing hormone receptor) have been reported to be the genetic defects responsible for an uncommon form of hyperthyroidism [Parma et al., Nature, 365: 649-651 (1993)] and for familial precocious puberty [Shenker et al. Nature, 365: 652-654 (1993)], respectively. In both cases the mutations result in constitutive activation of the 7TM receptors. Previously, other studies have shown that mutations that prevent the activation of 7TM receptors are responsible for states of hormone resistance which are responsible for diseases such as congenital nephrogenic diabetes insipidus. See Rosenthal et al., J. Biol. Chem., 268: 13030-13033 (1993). Still other studies have shown that several 7TM receptors can function as protooncogenes and be activated by mutational alteration. See, for example, Allen et al., Proc. Natl. Acad. Sci. USA, 88: 11354-11358 (1991) which suggests that spontaneously occuring mutations in some 7TM receptors may alter the normal function of the receptors and result in uncontrolled cell growth associated with human disease states such as neoplasia and atherosclerosis. Therefore, mutations in 7TM receptors may underlie a number of human pathologies.
Because a variety of therapeutic uses may be projected for 7TM receptors involved in immunological processes in both health and disease states and because it is generally believed that numerous proteins are involved in immunological processes, there continues to exist a need in the art for the identification of additional 7TM receptors that participate in such processes and especially a need for information specifically identifying and characterizing such proteins in terms of their amino acid sequence. Isolation of DNA encoding a novel 7TM receptor also provides the basis for determination of the role of receptor in health and disease states. To the extent that such receptors might form the basis for the development of therapeutic and/or diagnostic agents, it is essential that the DNA encoding them be isolated. The isolated DNA would, for example, provide for the large scale production of the 7TM proteins, allow for the identification of cells naturally producing them, and permit the preparation/identification of antibody substances and/or other novel binding substances (including natural ligands, agonists and antagonists) which are specifically reactive with a particular 7TM receptor (or group of receptors) and which have the capacity to modulate the biological activities of the receptor(s).